The present invention relates in general to communication systems and components, and is particularly directed to new and improved low pass filter-incorporating current mirror circuit architecture, that is readily suited for low voltage, low noise applications, such as, but not limited to subscriber line interface circuits.
Systems employed by telecommunication service providers contain what are known as subscriber line interface circuits or xe2x80x98SLIC""s, which interface communication signals with tip and ring leads of a wireline pair serving a relatively remote piece of subscriber communication equipment. In order to be interfaced with a variety of telecommunication circuits, including those providing codec functionality, present day SLICs must conform with a very demanding set of performance requirements, including accuracy, linearity, insensitivity to common mode signals, low noise, low power consumption, filtering, and ease of impedance matching programmability.
One of circuits commonly employed in a variety of SLIC implementations is a current mirror, the basic configuration of a typical one of which is schematically illustrated in FIG. 1. As shown therein, for a bipolar PNP transistor-based architecture, the current mirror comprises a current mirror input PNP transistor 10 and a current mirror output PNP transistor 20. The input transistor 10 has its base 11 coupled to receive a voltage reference VREF through a bias resistor 15, that is coupled to a (VCC) power supply rail 16. The collector 13 of transistor 10 is coupled to an input terminal IN, to which an input current Iin is supplied from an associated communication device interfaced therewith. The emitter 12 of the input transistor 10 is coupled through an emitter resistor 14 (having a resistance value R14) to the VCC supply rail 16.
In order to mirror the input current supplied to the input terminal IN, the base 11 of the current mirror input transistor 10 is coupled in common with the base 21 of current mirror output PNP transistor 20. It is also coupled to the emitter 32 of a base-offset PNP transistor 30, the collector 33 of which is coupled to a voltage reference terminal, such as ground (GND). The base 31 of PNP transistor 30 is coupled in common to the collector 13 of the current mirror input transistor 10. The emitter 22 of the current mirror output transistor 20 is coupled through an emitter resistor 24 (having a resistance value R24) to the (VCC) power supply rail 16, and its collector 23 is coupled to an output terminal OUT, from which a mirrored output current Iout is derived.
For the current mirror circuit architecture of FIG. 1 the following loop equation may be written:
IinR14/xcex110+Vbe10=IoutR24/xcex120+Vbe20.xe2x80x83xe2x80x83(1)
As the circuit of FIG. 1 is normally implemented with the input transistor 10 being well matched with the current mirror output transistor 20 (xcex110=xcex120, and Vbe10=Vbe20), and with resistors 14 and 24 being of equal resistance values (R14=R24), it follows that:
Iin=Iout.xe2x80x83xe2x80x83(2)
As pointed out above, among the performance requirements of present day SLICs is the need to provide (low pass) filtering. While this can be accomplished by means of a discrete filter circuit to which signal interface path with the SLIC is interfaced, such an approach increases the hardware complexity of the circuit and typically involves the use of power supply rails that provide sufficient overhead voltage. However, as designers of telecommunication integrated circuits, such as codecs and the like, continue to xe2x80x98lower the voltage supply rail barxe2x80x99 requirements for their devices (e.g., from five volts down to a voltage on the order of three volts or slightly higher), through the use of differential voltage-based implementations, the service provider is faced with the problem that such low voltages restrict voltage headroom.
With these restrictions in mind, the current mirror circuit of FIG. 1 is modified in accordance with the present invention to incorporate a relatively simple resistor-capacitor (RC) filter circuit in the base-coupling path of the input and output transistors, so as to realize a highly integrated low pass filter current mirror architecture, that not only reduces implementation complexity, but readily complies with the reduced power supply parameters of the SLIC.
In particular, a series resistor is substituted in place of the direct or common connection of the bases of the input and output transistors. In addition, a filter capacitor is coupled between the base of the output transistor and a the VCC supply rail. The effect of this RC circuit is to modify the transfer function of the current mirror to include a low pass filter, such that the output current is equal to the frequency content of the input current below the cut-off frequency as defined by the time constant of the RC filter.